In recent years, organic Electroluminescence (EL) displays including EL comprising an organic material have been attracting attention as a type of next-generation flat-panel displays succeeding liquid crystal displays. In display devices using active matrix method, such as organic EL displays, thin-film semiconductor devices for display apparatuses which are called thin-film transistors (TFT) (hereinafter, referred to also simply as “thin-film semiconductor devices”) are used.
In particular, unlike voltage-driven liquid crystal displays, the organic EL displays are current-driven display devices. Accordingly, there is an urgent need for development of thin-film semiconductor devices having excellent turn-On/turn-Off characteristics as driving circuits for the display devices using active matrix method.
Moreover, display devices are demanded to increase a screen size and reduce a cost. For thin-film semiconductor devices, bottom-gate thin-film semiconductor devices are generally used, because they have a gate electrode closer to a substrate more than a channel layer is, which reduces cost.
The bottom-gate thin-film semiconductor devices are classified into two major categories: channel-etching thin-film semiconductor devices in which a channel layer serving as a current flowing path is etched, and channel-protection (etching-stopper) thin-film semiconductor devices in which a channel layer is prevented from being etched.
In comparison to the channel-protection thin-film semiconductor devices, the channel-etching thin-film semiconductor devices can decrease steps of photolithography, thereby reducing a manufacturing cost.
On the other hand, the channel-protection thin-film semiconductor devices can prevent the channel layer from being damaged by etching, thereby suppressing the increase of characteristic variations on the surface of the substrate. Moreover, a channel layer of the channel-protection thin-film semiconductor devices can be manufactured to be thinner than that of the channel-etching thin-film semiconductor devices. As a result, the channel-protection thin-film semiconductor devices can reduce parasitic resistance components to improve the turn-On characteristics. The channel-protection thin-film semiconductor devices are therefore advantageous for high resolution.
Therefore, the channel-protection thin-film semiconductor devices are suitable as, for example, thin-film semiconductor devices in current-driven organic EL display devices having organic EL elements. Even if a manufacturing cost of the channel-protection thin-film semiconductor devices is higher than that of the channel-etching thin-film semiconductor devices, channel-protection thin-film semiconductor devices have been attempted to be applied to pixel circuits in organic EL display devices (for example, Non-Patent Literature 1).
Furthermore, as a channel-protection thin-film semiconductor device providing excellent turn-On characteristics, a thin-film semiconductor device including a channel layer having a raised part is disclosed (for example, Patent Literature 1). According to the technique disclosed in Patent Literature 1, non-raised parts on both sides of the raised part of the channel layer serving as a current flowing path is thinner than the raised part that is a higher part of the channel layer. Therefore, when a current flows between a source electrode and a drain electrode through the non-raised parts on the both sides of the raised part of the channel layer, resistance components can be reduced in a vertical direction in the channel layer. It is therefore possible to suppress crossing resistance through the non-raised parts on both sides of the raised part of the channel layer, thereby increasing ON-current. Furthermore, the upper portion of the raised part of the channel layer serves as a resistance between the source electrode and the drain electrode. Thereby, move of electric charges through a back channel between the source electrode and the drain electrode is suppressed.
In contrast, as a channel-protection thin-film semiconductor device achieving cost reduction, a thin-film semiconductor device having a channel protection layer that is a coated insulation film is disclosed (for example, Patent Literature 2). Patent Literature 2 discloses a method of forming a function layer included in the thin-film semiconductor device, by coating a liquid including desired material by a wet process. Therefore, it is possible to reduce a manufacturing cost of the display device and increase a throughput, in comparison to the conventional method of forming a function layer by a process in vacuum, such as Chemical Vapor Deposition (CVD) or spattering.